Liquid crystal compounds

ABSTRACT

A liquid crystal compound represented by formula (I): ##STR1## wherein R 1  represents an alkyl group having from 3 to 18 carbon atoms; R 2  represents an alkyl group having from 3 to 18 carbon atoms; X represents ##STR2## Y represents ##STR3## (A) and (B) each represents a group selected from the group consisting of ##STR4## and ##STR5## and * indicates an optically active center.

This is a divisional of application Ser. No. 07/557,845 filed Jul. 25,1990, now U.S. Pat. No. 5,207,946.

FIELD OF THE INVENTION

This invention relates to ferroelectric chiral smectic liquid crystalcompounds which are suitable for use in electro-optical display devices.This invention also relates to ferroelectric liquid crystal compoundsexhibiting three stable molecular orientation states which are suitablefor use in display elements or electro-optic elements utilizing aresponse to an electric field.

BACKGROUND OF THE INVENTION

Electro-optic devices using liquid crystals which have hitherto beendeveloped and put into practical use include those using nematic liquidcrystals, such as a DSM mode, a TN mode, a G-H mode, and an STN mode.However, any of these devices using nematic liquid crystals has a veryslow electro-optic response requiring a switching time from several toseveral tens milliseconds and is hence limited in range of application.The slow response of the elements using nematic liquid crystals isbecause the torque of moving molecules, basically being based onanisotropy of dielectric constant, is not so strong. Under suchcircumstances, Meyer, et al. developed ferroelectric liquid crystalswhich undergo spontaneous polarization (Ps) and has a strong torque, thetorque being based on Ps×E (applied electric field), and thereby has ahigh-speed response in the order of microseconds as disclosed in LeJournal de Physique, Vol. 36, L-69 (1975). Further, JP-A-63-307837 (theterm "JP-A" as used herein means an "unexamined published Japanesepatent application") discloses new ferroelectric liquid crystals, butgives no description about "three states" hereinafter described.

Several high-speed electro-optic devices using ferroelectric liquidcrystals have already been proposed. Typically included in such devicesis an element in which a twisted structure is untwisted by the force ofa wall surfaces, and two molecular alignment layers in parallel in thewall surface are varied by polarity of the applied electric field asdescribed, e.g., in JP-A-56-107216.

Existence of a compound showing ideal two states having an electricfield response waveform as shown in FIG. 1 is prerequisite torealization of the above-described device. However, such a compoundexhibiting ideal two states has not yet been discovered. Any of the sofar synthesized bistable liquid crystals has a response waveform asshown in FIG. 2 but not that of FIG. 1. When the state-of-the-art liquidcrystals having a response waveform of FIG. 2 are used, for example, inlight switching circuits, since a transmission gradually changesaccording as an applied voltage changes from negative to positive, thepurpose cannot be sufficiently accomplished simply by changing theapplied voltage between "on" and "off". Moreover, so far synthesizedbistable liquid crystals find difficulty in making a mono-domain statein its Sc* phase with no voltage applied, i.e., an ideal molecularorientation state, easily causing defect or a molecular orientationdisturbance called twist. Thus, it has been difficult to realize theabove-stated ideal two states of molecular orientation over a wide area.Further, because the threshold value (voltage at which a luminancechanges by a prescribed value) is low, dynamic driving is liable tosuffer from reduction in contrast or reduction in viewing angle.Furthermore, these conventional bistable liquid crystals do not exhibithysteresis as shown in FIG. 1 but that shown in FIG. 2 so that they haveno memory effect. Therefore, it is necessary to continue applying avoltage of V₃ of FIG. 2 or applying a high frequency for the liquidcrystal to maintain a stable response in the Sc* phase, which, in eithercase, entails a considerable energy loss.

Thus, the conventional electro-optic devices have many problems waitingfor solution notwithstanding the strong demand for making effective useof the strong connection between an applied electric field and molecularorientation exhibited by ferroelectric liquid crystals.

SUMMARY OF THE INVENTION

An object of this invention is to provide a novel liquid crystalcompound which exhibits a stable molecular orientation state having ahigh light/shade contrast with no electric field applied, has clearthreshold characteristics and clear hysteresis as shown in FIG. 3,easily realizes dynamic driving, and is applicable to liquid crystalelectro-optic devices utilizing three states which make it possible toobtain a high-speed response.

More specifically, an object of this invenion is to provide a novelferroelectric liquid crystal exhibiting a chiral smectic (Sc*) phase.

Another object of this invention is to provide a novel ferroelectricliquid crystal exhibiting three states which are entirely different froma chiral semctic C phase (Sc* phase), a conventional bistable statephase.

The terminology "three states" as used herein means three stable statesof molecular orientation explained below. In a liquid crystalelectro-optic device comprising a pair of electrode substrates with aprescribed gap therebetween and a ferroelectric liquid crystal beingsandwiched between the pair of substrates, the electrodes beingconnected to an electric power source so that a voltage of triangularwave as shown in FIG. 4 (A) is applied, the ferroelectric liquid crystalshows a first stable state of molecular orientation as shown by numeral2 of FIG. 4(D) with no electric field applied, a second stable state ofmolecular orientation as shown by numeral 1 of FIG. 4(D) differing fromthe first stable state with an electric field applied to one direction,and a third stable state of molecular orientation as shown by numeral 3of FIG. 4(D) differing from either of the first and second stable stateswith an electric field applied to another direction. With respect toliquid crystal electro-optic devices utilizing these three stablestates, the inventors have already filed a Japanese patent applicationNo. 70212/88.

To the contrary, commercially available nematic liquid crystals and sofar synthesized bistable liquid crystals have no such three stablestates as revealed in FIGS. 4(B) and (C), respectively.

The above-stated ferroelectric liquid crystals having three states(hereinafter sometimes referred to as tristable liquid crystals)according to the present invention produce striking effects when appliedto liquid crystal displays as compared with the conventional nematicliquid crystals as follows.

While the conventional liquid crystals should be driven through a verycomplicated system called an active matrix system, the tristableferroelectric liquid crystal can be driven through a simple matrixdisplay. Accordingly, the display element using the tristableferroelectric liquid crystal can be produced by easy steps and makes itfeasible to widen the display area and to reduce the production cost,whereas the conventional display elements require complicated productionsteps, encounter difficulty in widening the display area, and incur highproduction cost.

That is, the present invention provides a liquid crystal compoundrepresented by formula (I): ##STR6## wherein R₁ represents an alkylgroup having from 3 to 18 carbon atoms; R₂ represents an alkyl grouphaving from 3 to 18 carbon atoms; X represents ##STR7## Y represents##STR8## (A) and (B) each represents a group selected from the groupconsisting of ##STR9## and ##STR10## and * indicates an optically activecenter.

The present invention also provides a liquid crystal compoundrepresented by formula (I), wherein R₁ represents an alkyl group havingfrom 5 to 18 carbon atoms; R₂ represents an alkyl group having from 4 to15 carbon, which exhibits three stable states.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, and 3 each shows hysteresis of an ideal bistable liquidcrystal which is not actually available, a conventional crystal which isnot actually available, a conventionally developed bistable liquidcrystal, and a tristable liquid crystal according to the presentinvention, respectively, in which an applied voltage is plotted asabscissa and a transmission (%) as ordinate.

FIG. 4(A) shows a triangular wave applied.

FIGS. 4(B), (C), and (D), each shows an electro-optic response of acommercially available nematic liquid crystal, a conventionallysynthesized bistable liquid crystal, and a tristable liquid crystalaccording to the present invention, respectively.

FIG. 5 shows an electro-clinic effect, in which (a) is an alternatingvoltage applied to a liquid crystal electro-optic element, and (b) showschanges of transmission with the alternating voltage (a).

FIG. 6 illustrates switching among the three states of the compoundaccording to the present invention, in which (a) is a triangular voltagewave applied to a liquid crystal electro-optic element, (b) is apolarization inversion current; and (c) is a transmission changing withthe voltage (a).

DETAILED DESCRIPTION OF THE INVENTION

Specific examples of preferred compounds represented by formula (I) areshown below. ##STR11## wherein R₁ and R₂ are as defined above.

In formula (I) and the above-illustrated formulae, R₁ preferablyrepresents an alkyl group having from 5 to 18 carbon atoms, and morepreferably a straight chain alkyl group having from 6 to 12 carbonatoms. R₂ preferably represents an alkyl group having from 4 to 15carbon atoms, and more preferably a straight chain alkyl group havingfrom 5 to 12 carbon atoms.

Synthesis examples of the compounds according to the present inventionare shown below.

SYNTHESIS EXAMPLE 1

4-Benzyloxybenzoic acid chloride and an optically active1,1,1-trifluoro-2-alkanol were reacted to obtain a 4-benzyloxybenzoicacid 1,1,1-trifluoro-2-alkyl ester. The ester was hydrogenated to obtaina 4-hydroxybenzoic acid 1,1,1-trifluoro-2-alkyl ester. The resultingalkyl ester was then reacted with a 4-n-alkanoyloxyphenyl(orbiphenyl)carboxylic acid chloride to obtain a desired compound, anoptically active 4-(1,1,1-trifluoro-2-alkyloxycarbonyl)phenyl4-n-alkanoyloxybenzoate (or biphenylcarboxylate).

SYNTHESIS EXAMPLE 2

Terephthalic acid chloride was reacted with an optically active1,1,1-trifluoro-2-alkanol to obtain a terephalic acid1,1,1-trifluoroalkyl monoester, which was then reacted with a4-alkanoyloxy-4'-hydroxybiphenyl to obtain an optically active4-alkanoyloxybiphenyl 4-(1,1,1-trifluoro-2-alkyloxycarbonyl)benzoate.

The present invention is now illustrated in greater detail by way ofExamples, but it should be understood that the present invention is notdeemed to be limited thereto. In Examples, phase transition points ofliquid crystals were measured by means of DSC (differential scanningcalorimeter) combined with microscopic observation on hot stage.

EXAMPLE 1 1) Synthesis of 1,1,1-Trifluoro-2-decyl 4-Benzyloxybenzoate##STR12##

In 10 ml of methylene chloride was dissolved 1.23 g of4-benzyloxybenzoic acid chloride, and a solution of 0.96 g of opticallyactive 1,1,1-trifluoro-2-decanol, 0.55 g of dimethylaminopyridine, and0.48 g of triethylamine in 20 ml of methylene chloride was added theretoin small portions under ice-cooling.

The reaction mixture was allowed to warm to room temperature and wasreacted at that temperature for one day. The reaction mixture was pouredinto ice-water and extracted with methylene chloride. The methylenechloride phase was washed successively with diluted hydrochloric acid,water, a 1N sodium carbonate aqueous solution, and water and dried overanhydrous magnesium sulfate. The solvent was removed by distillation toobtain a crude product. The crude product was purified by columnchromatography using silica gel and toluene and then recrystallizationfrom ethanol to obtain 1.84 g of the entitled compound.

2) Synthesis of 1,1,1-Trifluoro-2-decyl 4-Hydroxybenzoate ##STR13##

The compound obtained in 1) above was dissolved in 15 ml of ethanol, and0.36 g of 10% palladium-on-carbon was added to solution to conducthydrogenation in a hydrogen atmosphere to obtain 1.43 g of the entitledcompound.

3) Synthesis of 4-(1,1,1-Trifluoro-2-decyloxycarbonyl)phenyl4'-n-Nonanoyloxybiphenyl-4-carboxylate ##STR14##

4'-n-Nonanoyloxybiphenyl-4-carboxylic acid (1.20 g) was refluxedtogether with an excess of thionyl chloride for 6 hours. The unreactedthionyl chloride was removed by distillation to obtain4'-n-nonanoyloxydiphenylcarboxylic acid chloride.

The resulting acid chloride was dissolved in 12.0 ml of methylenechloride, and a solution of 1.00 g of the above-prepared1,1,1-trifluorodecyl ester, 0.32 g of triethylamine, and 0.37 g ofdimethylaminopyridine in 30 ml of methylene chloride was slowly addedthereto under ice-cooling, and the mixture was allowed to react at roomtemperature for one day.

The reaction mixture was poured into ice-water and extracted withmethylene chloride. The extract was washed successively with dilutedhydrochloric acid, water, a sodium carbonate aqueous solution, and waterand dried over anhydrous sodium sulfate. The solvent was removed bydistillation to obtain a crude product. The crude product was purifuedby column chromatography using silica gel and toluene to obtain 1.1 g ofthe desired optically active compound.

Phase transition points of the resulting compound were determined afterrecrystallization from absolute ethanol. ##STR15##

EXAMPLE 2 Synthesis of 4-n-(1,1,1-Trifluoro-2-octyloxy-carbonyl)phenyl4'-n-Nonanoyloxybiphenyl-4-carboxylate ##STR16##

The entitled optically active compound was synthesized in the samemanner as in Example 1, except for replacing 1,1,1-trifluoro-2-decanolas used in 1) of Example 1 with 1,1,1-trifluoro-2-octanol.

Phase transition temperature of the resulting compound was determined byDSC and microscopic measurements. ##STR17##

EXAMPLE 3 Synthesis of 4-n-(1,1,1-Trifluoro-2-octyloxycarbonyl)-phenyl4'-n-Decyloxycarbonylbiphenyl-4-carboxylate ##STR18##

The entitled optically active compound was synthesized in the samemanner as in Example 1, except for replacing4'-nonanoyloxybiphenyl-4-carboxylic acid as used in 3) of Example 1 with4'-decyloxycarbonylbiphenyl-4-carboxylic acid.

Phase transition temperature of the resulting compound was determined byDSC and microscopic measurements. ##STR19##

EXAMPLE 4 Synthesis of4'-n-Undecanoyloxybiphenyl-4-(1,1,1-Trifluoro-2-octyloxycarbonyl)benzoate##STR20## 1) Synthesis of 1,1,1-Trifluoro-2-octylterephthalic ##STR21##

In 50 ml of methylene chloride was dissolved 11.2 g of terephthalic acidchloride, and 10.0 g of optically active 1,1,1-trifluoro-2-octanol and13 g of pyridine were added dropwise to the solution under ice-cooling.The reaction mixture was allowed to warm to room temperature, and thereaction was continued at that temperature for one day. The reactionmixture was washed successively with diluted hydrochloric acid, water, a1N sodium bicarbonate aqueous solution, and water. The organic layercollected was dried over anhydrous magnesium sulfate, and the solventwas removed by distillation under reduced pressure. The resulting crudeproduct was purified by column chromatography using silica gel andtoluene and then recrystallization from ethanol to obtain 3.3 g of theentitled compound.

2) Synthesis of 4'-n-Undecanoyloxybiphenyl-(4)4-(1,1,1-Trifluoro-2-octyloxycarbonyl)benzoate

To 100 ml of a tetrahydrofuran solvent were added 3.3 g of the1,1,1-trifluoro-2-octylterephthalic acid monoester obtained in 1) above,3.5 g of 4-undecanoyloxy-4'-hydroxybiphenyl, 3.19 g ofdicyclohexylcarbodiimide, and 0.3 g of dimethylaminopyridine, and themixture was allowed to react at room temperature for one day.

After the tetrahydrofuran solvent in the reaction mixture wasappropriately reduced by vacuum distillation, the mixture was pouredinto cool water and extracted with methylene chloride. The extract wasthoroughly washed successively with an aqueous 1N sodium bicarbonatesolution, water, diluted hydrochloric acid, and water until the washingbecame neutral and dried over anhydrous sodium sulfate. The solvent wasremoved by distillation, and the resulting crude product was purified bycolumn chromatography using silica gel and toluene and then repeatedrecrystallization from ethanol to obtain 1.5 g of an optically activedesired compound.

     α!.sub.D.sup.20 =+23.8° ##STR22##

EXAMPLE 5 Synthesis of 4-n-Decyloxycarbonylphenyl4'-(1,1,1-Trifluoro-2-octyloxycarbonyl)biphenyl-4-carboxylate ##STR23##

4-(1,1,1-Trifluoro-2-octyloxycarbonyl)-4'-hydroxybiphenyl (0.5 g) and0.4 g of n-4-decyloxycarbonylbenzoic acid were reacted in 30 ml oftetrahydrofuran in the presence of 0.3 g of dicyclocarbodiimide and afew pieces of dimethylaminopyridine. The resulting crude product waspurified by silica gel column chromatography using a 10:0.5 (by volume)mixture of hexane and ethyl acetate as an eluent and thenrecrystallization from ethanol to obtain 0.1 g of the entitled opticallyactive compound. ##STR24##

EXAMPLE 6

The liquid crystal compound obtained in Example 1 was filled while in anisotropic phase into a liquid crystal cell having a polyimideorientation film having been subjected to a rubbing treatment on an ITO(indium tin oxide) electrode substrate (cell thickness: 2.9 μm).

The resulting cell was slowly cooled at a rate of 0.1 to 1.0° C./min toorientate the liquid crystal molecules in an S_(A) phase. A square wavevoltage of ±30 V and 10 Hz was applied, and an electro-optic responsewas detected with a polarizing microscope equipped with aphotomultiplier. As a result, an electro-clinic effect (b) opticallyresponding to the applied electric field (a) was observed in the S_(A)phase. The same effect was observed in liquid crystal cells obtained byusing other liquid crystal compounds prepared in the foregoing Examples.

EXAMPLE 7

The liquid crystal cell prepared in the same manner as in Example 6 wasset in a polarizing microscope equipped with a photomultiplier having apair of polarizing sheets crosswise in such a manner that the molecularlonger axis direction and the polarizer were at an angle of 22.5°. Then,the cell was gradually cooled at a rate of 0.1° to 1.0° C./min until itexhibited an S(3)* phase. The cell was further cooled, and in atemperature range of from 95.0° to 10.0° C., a triangular wave voltage(a) of ±30 V and 10 Hz was applied. As shown in FIG. 6, the transmission(c) changed into three states, i.e., a shade state with a minus voltageapplied, an intermediate state with a voltage of zero, and a light statewith a plus voltage applied; and the polarization inversion currentwaveform (b) also showed peaks corresponding to these changes. That is,the liquid crystal molecules proved to exhibit three stable states oforientation. The similar effect was observed in liquid crystal cellsusing other liquid crystal compounds obtained in the foregoing Examples.

As described above, the novel liquid crystal compounds of the presentinvention exhibit the conventional bistable states or three stablestates and can be utilized in a wide range of application, such asdisplay devices and switching devices.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A liquid crystal compound represented by formula(IV): ##STR25## wherein R₁ represents an alkyl group having from 3 to 18carbon atoms, R₂ represents an alkyl group having from 4 to 15 carbonatoms, and the compound exhibits tristability when in an S*.sub.(3)phase.
 2. A liquid crystal compound represented by formula (V):##STR26## wherein R₁ represents an alkyl group having from 3 to 18carbon atoms, R₂ represents an alkyl group having from 4 to 15 carbonatoms, and the compound exhibits tristability when in an S*.sub.(3)phase.